Excited state management

ABSTRACT

Arrangements and techniques for providing organic emissive layers are provided, in which the emissive layer includes a first dopant having a dissociative energy level. A second dopant in the emissive layer provides a solid state sink energy level, to which doubly excited excitons and/or polarons may transition instead of to the dissociative energy level, thereby decreasing the undesirable effects of transitions to the dissociative energy level.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNos. 62/003,591, filed May 28, 2014; 62/095,904, filed Dec. 23, 2014;62/103,483, filed Jan. 14, 2015; and 62/108,100, filed Jan. 27, 2015,the entire contents of each of which is incorporated herein byreference.

STATEMENT REGARDING GOVERNMENT RIGHTS

This invention was made with government support under Contract No. DOEDESC00001013 awarded by the Department of Energy, and underFA9550-14-1-0245 awarded by the U.S. Air Force Office of ScientificResearch. The government has certain rights in the invention.

PARTIES TO A JOINT RESEARCH AGREEMENT

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to a joint universitycorporation research agreement: Regents of the University of Michigan,Princeton University, University of Southern California, and theUniversal Display Corporation. The agreement was in effect on and beforethe date the claimed invention was made, and the claimed invention wasmade as a result of activities undertaken within the scope of theagreement.

FIELD OF THE INVENTION

The present invention relates to organic light emitting devices (OLEDs)and, more specifically, to arrangements and techniques that providerelatively high performance and lifetime to various types and componentsof OLEDs, and devices such as organic light emitting diodes and otherdevices, including the same.

BACKGROUND

Opto-electronic devices that make use of organic materials are becomingincreasingly desirable for a number of reasons. Many of the materialsused to make such devices are relatively inexpensive, so organicopto-electronic devices have the potential for cost advantages overinorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Examples of organic opto-electronic devices include organic lightemitting devices (OLEDs), organic phototransistors, organic photovoltaiccells, and organic photodetectors. For OLEDs, the organic materials mayhave performance advantages over conventional materials. For example,the wavelength at which an organic emissive layer emits light maygenerally be readily tuned with appropriate dopants.

OLEDs make use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as flat paneldisplays, illumination, and backlighting. Several OLED materials andconfigurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and5,707,745, which are incorporated herein by reference in their entirety.

One application for phosphorescent emissive molecules is a full colordisplay. Industry standards for such a display call for pixels adaptedto emit particular colors, referred to as “saturated” colors. Inparticular, these standards call for saturated red, green, and bluepixels. Color may be measured using CIE coordinates, which are wellknown to the art.

One example of a green emissive molecule is tris(2-phenylpyridine)iridium, denoted Ir(ppy)₃, which has the following structure:

In this, and later figures herein, we depict the dative bond fromnitrogen to metal (here, Ir) as a straight line.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules.

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. Where a first layer isdescribed as “disposed over” a second layer, the first layer is disposedfurther away from substrate. There may be other layers between the firstand second layer, unless it is specified that the first layer is “incontact with” the second layer. For example, a cathode may be describedas “disposed over” an anode, even though there are various organiclayers in between.

As used herein, “solution processible” means capable of being dissolved,dispersed, or transported in and/or deposited from a liquid medium,either in solution or suspension form.

A ligand may be referred to as “photoactive” when it is believed thatthe ligand directly contributes to the photoactive properties of anemissive material. A ligand may be referred to as “ancillary” when it isbelieved that the ligand does not contribute to the photoactiveproperties of an emissive material, although an ancillary ligand mayalter the properties of a photoactive ligand.

As used herein, and as would be generally understood by one skilled inthe art, a first “Highest Occupied Molecular Orbital” (HOMO) or “LowestUnoccupied Molecular Orbital” (LUMO) energy level is “greater than” or“higher than” a second HOMO or LUMO energy level if the first energylevel is closer to the vacuum energy level. Since ionization potentials(IP) are measured as a negative energy relative to a vacuum level, ahigher HOMO energy level corresponds to an IP having a smaller absolutevalue (an IP that is less negative). Similarly, a higher LUMO energylevel corresponds to an electron affinity (EA) having a smaller absolutevalue (an EA that is less negative). On a conventional energy leveldiagram, with the vacuum level at the top, the LUMO energy level of amaterial is higher than the HOMO energy level of the same material. A“higher” HOMO or LUMO energy level appears closer to the top of such adiagram than a “lower” HOMO or LUMO energy level.

As used herein, and as would be generally understood by one skilled inthe art, a first work function is “greater than” or “higher than” asecond work function if the first work function has a higher absolutevalue. Because work functions are generally measured as negative numbersrelative to vacuum level, this means that a “higher” work function ismore negative. On a conventional energy level diagram, with the vacuumlevel at the top, a “higher” work function is illustrated as furtheraway from the vacuum level in the downward direction. Thus, thedefinitions of HOMO and LUMO energy levels follow a different conventionthan work functions.

More details on OLEDs, and the definitions described above, can be foundin U.S. Pat. No. 7,279,704, which is incorporated herein by reference inits entirety.

SUMMARY OF THE INVENTION

According to an embodiment, an organic emissive layer is provided thatincludes a host material, a first dopant having a dissociative firstenergy level at which doubly excited excitons and/or polarons undergo adissociative reaction, and a second dopant having a solid state sinkenergy level that between a singly-excited energy level of the firstdopant and a multiply-excited energy level of the first dopant.Alternatively or in addition, the solid state sink energy level may belower than the dissociative energy level. The multiply-excited state maybe, for example, a doubly-excited triplet energy level or adoubly-excited polaron energy level of the first dopant. The solid statesink energy level may be at least 0.2 eV lower than the multiply-excitedstate of the first dopant. The rate of transition of the excitons and/orpolarons from a doubly-excited triplet energy level of the first dopantto the solid state sink energy level of the second dopant may be higherthan the rate of transition from the double-excited triplet energy levelof the first dopant to the dissociative energy level. The first dopantmay have an emissive first triplet energy level that is lower than thesolid state sink energy level of the second dopant. The concentration ofthe first dopant may be graded within the host material. The firstdopant may be a thermally assisted delayed fluorescence dopant. Thesecond dopant may comprise a material that absorbs light in theultraviolet region of the electromagnetic spectrum, such as NTCDA. Thefirst dopant may include a phosphorescent dopant or a fluorescentdopant. The dopant may provide thermally assisted delayed fluorescencein the emissive layer.

In an embodiment, a device is provided that includes an emissive layeras previously described. The device may include an OLED, such as awhite-emitting OLED (WOLED). The emissive layer may emit any color oflight, such as light in the blue region of the visible spectrum.

According to an embodiment, a first device comprising a first organiclight emitting device is also provided. The first organic light emittingdevice can include an anode, a cathode, and an organic layer, disposedbetween the anode and the cathode. The organic layer can include a firstdopant having a dissociative first energy level at which doubly excitedexcitons and/or polarons undergo a dissociative reaction, and a seconddopant having a solid state sink energy level that is lower than thedissociative energy level. The first device can be a consumer product,an organic light-emitting device, and/or a lighting panel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an organic light emitting device.

FIG. 2 shows an inverted organic light emitting device that does nothave a separate electron transport layer.

FIGS. 3A-3B show two mechanisms resulting in PHOLED failure via thedissociation of molecular species into non-emissive and permanent trapstates; FIG. 3A shows exciton-polaron, dissociation; FIG. 3B showsexciton-exciton annihilation.

FIG. 4 shows an example energy level arrangement including a solid statesink energy level as disclosed herein.

FIG. 5A shows a structure suitable to measure aza-substitution in an H3Pmaterial.

FIG. 5B shows features of the compounds resulting from the structure inFIG. 5A.

DETAILED DESCRIPTION

Generally, an OLED comprises at least one organic layer disposed betweenand electrically connected to an anode and a cathode. When a current isapplied, the anode injects holes and the cathode injects electrons intothe organic layer(s). The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, an “exciton,” which is a localizedelectron-hole pair having an excited energy state, is formed. Light isemitted when the exciton relaxes via a photoemissive mechanism. In somecases, the exciton may be localized on an excimer or an exciplex.Non-radiative mechanisms, such as thermal relaxation, may also occur,but are generally considered undesirable.

The initial OLEDs used emissive molecules that emitted light from theirsinglet states (“fluorescence”) as disclosed, for example, in U.S. Pat.No. 4,769,292, which is incorporated by reference in its entirety.Fluorescent emission generally occurs in a time frame of less than 10nanoseconds.

More recently, OLEDs having emissive materials that emit light fromtriplet states (“phosphorescence”) have been demonstrated. Baldo et al.,“Highly Efficient Phosphorescent Emission from OrganicElectroluminescent Devices,” Nature, vol. 395, 151-154, 1998;(“Baldo-I”) and Baldo et al., “Very high-efficiency green organiclight-emitting devices based on electrophosphorescence,” Appl. Phys.Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporatedby reference in their entireties. Phosphorescence is described in moredetail in U.S. Pat. No. 7,279,704 at cols. 5-6, which are incorporatedby reference.

FIG. 1 shows an organic light emitting device 100. The figures are notnecessarily drawn to scale. Device 100 may include a substrate 110, ananode 115, a hole injection layer 120, a hole transport layer 125, anelectron blocking layer 130, an emissive layer 135, a hole blockinglayer 140, an electron transport layer 145, an electron injection layer150, a protective layer 155, a cathode 160, and a barrier layer 170.Cathode 160 is a compound cathode having a first conductive layer 162and a second conductive layer 164. Device 100 may be fabricated bydepositing the layers described, in order. The properties and functionsof these various layers, as well as example materials, are described inmore detail in U.S. Pat. No. 7,279,704 at cols. 6-10, which areincorporated by reference.

More examples for each of these layers are available. For example, aflexible and transparent substrate-anode combination is disclosed inU.S. Pat. No. 5,844,363, which is incorporated by reference in itsentirety. An example of a p-doped hole transport layer is m-MTDATA dopedwith F₄-TCNQ at a molar ratio of 50:1, as disclosed in U.S. PatentApplication Publication No. 2003/0230980, which is incorporated byreference in its entirety. Examples of emissive and host materials aredisclosed in U.S. Pat. No. 6,303,238 to Thompson et al., which isincorporated by reference in its entirety. An example of an n-dopedelectron transport layer is BPhen doped with Li at a molar ratio of 1:1,as disclosed in U.S. Patent Application Publication No. 2003/0230980,which is incorporated by reference in its entirety. U.S. Pat. Nos.5,703,436 and 5,707,745, which are incorporated by reference in theirentireties, disclose examples of cathodes including compound cathodeshaving a thin layer of metal such as Mg:Ag with an overlyingtransparent, electrically-conductive, sputter-deposited ITO layer. Thetheory and use of blocking layers is described in more detail in U.S.Pat. No. 6,097,147 and U.S. Patent Application Publication No.2003/0230980, which are incorporated by reference in their entireties.Examples of injection layers are provided in U.S. Patent ApplicationPublication No. 2004/0174116, which is incorporated by reference in itsentirety. A description of protective layers may be found in U.S. PatentApplication Publication No. 2004/0174116, which is incorporated byreference in its entirety.

FIG. 2 shows an inverted OLED 200. The device includes a substrate 210,a cathode 215, an emissive layer 220, a hole transport layer 225, and ananode 230. Device 200 may be fabricated by depositing the layersdescribed, in order. Because the most common OLED configuration has acathode disposed over the anode, and device 200 has cathode 215 disposedunder anode 230, device 200 may be referred to as an “inverted” OLED.Materials similar to those described with respect to device 100 may beused in the corresponding layers of device 200. FIG. 2 provides oneexample of how some layers may be omitted from the structure of device100.

The simple layered structure illustrated in FIGS. 1 and 2 is provided byway of non-limiting example, and it is understood that embodiments ofthe invention may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional OLEDs may be achieved by combining the various layersdescribed in different ways, or layers may be omitted entirely, based ondesign, performance, and cost factors. Other layers not specificallydescribed may also be included. Materials other than those specificallydescribed may be used. Although many of the examples provided hereindescribe various layers as comprising a single material, it isunderstood that combinations of materials, such as a mixture of host anddopant, or more generally a mixture, may be used. Also, the layers mayhave various sublayers. The names given to the various layers herein arenot intended to be strictly limiting. For example, in device 200, holetransport layer 225 transports holes and injects holes into emissivelayer 220, and may be described as a hole transport layer or a holeinjection layer. In one embodiment, an OLED may be described as havingan “organic layer” disposed between a cathode and an anode. This organiclayer may comprise a single layer, or may further comprise multiplelayers of different organic materials as described, for example, withrespect to FIGS. 1 and 2.

Structures and materials not specifically described may also be used,such as OLEDs comprised of polymeric materials (PLEDs) such as disclosedin U.S. Pat. No. 5,247,190 to Friend et al., which is incorporated byreference in its entirety. By way of further example, OLEDs having asingle organic layer may be used. OLEDs may be stacked, for example asdescribed in U.S. Pat. No. 5,707,745 to Forrest et al, which isincorporated by reference in its entirety. The OLED structure maydeviate from the simple layered structure illustrated in FIGS. 1 and 2.For example, the substrate may include an angled reflective surface toimprove out-coupling, such as a mesa structure as described in U.S. Pat.No. 6,091,195 to Forrest et al., and/or a pit structure as described inU.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated byreference in their entireties.

Unless otherwise specified, any of the layers of the various embodimentsmay be deposited by any suitable method. For the organic layers,preferred methods include thermal evaporation, ink-jet, such asdescribed in U.S. Pat. Nos. 6,013,982 and 6,087,196, which areincorporated by reference in their entireties, organic vapor phasedeposition (OVPD), such as described in U.S. Pat. No. 6,337,102 toForrest et al., which is incorporated by reference in its entirety, anddeposition by organic vapor jet printing (OVJP), such as described inU.S. Pat. No. 7,431,968, which is incorporated by reference in itsentirety. Other suitable deposition methods include spin coating andother solution based processes. Solution based processes are preferablycarried out in nitrogen or an inert atmosphere. For the other layers,preferred methods include thermal evaporation. Preferred patterningmethods include deposition through a mask, cold welding such asdescribed in U.S. Pat. Nos. 6,294,398 and 6,468,819, which areincorporated by reference in their entireties, and patterning associatedwith some of the deposition methods such as ink jet and OVJD. Othermethods may also be used. The materials to be deposited may be modifiedto make them compatible with a particular deposition method. Forexample, substituents such as alkyl and aryl groups, branched orunbranched, and preferably containing at least 3 carbons, may be used insmall molecules to enhance their ability to undergo solution processing.Substituents having 20 carbons or more may be used, and 3-20 carbons isa preferred range. Materials with asymmetric structures may have bettersolution processibility than those having symmetric structures, becauseasymmetric materials may have a lower tendency to recrystallize.Dendrimer substituents may be used to enhance the ability of smallmolecules to undergo solution processing.

Devices fabricated in accordance with embodiments of the presentinvention may further optionally comprise a barrier layer. One purposeof the barrier layer is to protect the electrodes and organic layersfrom damaging exposure to harmful species in the environment includingmoisture, vapor and/or gases, etc. The barrier layer may be depositedover, under or next to a substrate, an electrode, or over any otherparts of a device including an edge. The barrier layer may comprise asingle layer, or multiple layers. The barrier layer may be formed byvarious known chemical vapor deposition techniques and may includecompositions having a single phase as well as compositions havingmultiple phases. Any suitable material or combination of materials maybe used for the barrier layer. The barrier layer may incorporate aninorganic or an organic compound or both. The preferred barrier layercomprises a mixture of a polymeric material and a non-polymeric materialas described in U.S. Pat. No. 7,968,146, PCT Pat. Application Nos.PCT/US2007/023098 and PCT/US2009/042829, which are herein incorporatedby reference in their entireties. To be considered a “mixture”, theaforesaid polymeric and non-polymeric materials comprising the barrierlayer should be deposited under the same reaction conditions and/or atthe same time. The weight ratio of polymeric to non-polymeric materialmay be in the range of 95:5 to 5:95. The polymeric material and thenon-polymeric material may be created from the same precursor material.In one example, the mixture of a polymeric material and a non-polymericmaterial consists essentially of polymeric silicon and inorganicsilicon.

Devices fabricated in accordance with embodiments of the invention canbe incorporated into a wide variety of electronic component modules (orunits) that can be incorporated into a variety of electronic products orintermediate components. Examples of such electronic products orintermediate components include display screens, lighting devices suchas discrete light source devices or lighting panels, etc. that can beutilized by the end-user product manufacturers. Such electroniccomponent modules can optionally include the driving electronics and/orpower source(s). Devices fabricated in accordance with embodiments ofthe invention can be incorporated into a wide variety of consumerproducts that have one or more of the electronic component modules (orunits) incorporated therein. Such consumer products would include anykind of products that include one or more light source(s) and/or one ormore of some type of visual displays. Some examples of such consumerproducts include flat panel displays, computer monitors, medicalmonitors, televisions, billboards, lights for interior or exteriorillumination and/or signaling, heads-up displays, fully or partiallytransparent displays, flexible displays, laser printers, telephones,cell phones, tablets, phablets, personal digital assistants (PDAs),laptop computers, digital cameras, camcorders, viewfinders,micro-displays, 3-D displays, smart watches, vehicles, a large areawall, theater or stadium screen, or a sign. Various control mechanismsmay be used to control devices fabricated in accordance with the presentinvention, including passive matrix and active matrix. Many of thedevices are intended for use in a temperature range comfortable tohumans, such as 18 C to 30 C, and more preferably at room temperature(20-25 C), but could be used outside this temperature range, forexample, from −40 C to +80 C.

Generally, different types, arrangements, and colors of OLEDs may havedramatically different operational lifetimes. For example, bluephosphorescent OLEDs (PHOLEDs) often have a relatively short operationallifetime. Blue emission comprises about 15-25% of the emission in whiteOLEDs (WOLEDs) depending on the WOLED color coordinates and the colortemperature of the lighting. Since the blue PHOLED lifetime typically isfar shorter than that of red and green, it therefore also presents theprimary limitation to the WOLED lifetime.

The most fundamental source of degradation in PHOLEDs is the presence ofa high density of energetic molecular excited states, or excitons, whichmay dissipate their energy by breaking molecular bonds, therebydestroying the active electronic molecular species and at the same timeforming a non-radiative defect. Particularly, excitons on the phosphorcan encounter free electrons (i.e. “electron-polarons”) or excitons onthe host molecules, resulting in an instantaneous doubling of thepolaron (or exciton) energy as shown, for example, in FIGS. 3A-3B. FIGS.3A-3B show two mechanisms resulting in PHOLED failure via thedissociation of molecular species into non-emissive and permanent trapstates, R (dashed line)—exciton polaron, i.e. charge, as shown in FIG.3A, and exciton-exciton annihilation, as shown in FIG. 3B. Solid linescorrespond to the molecular ground (S0) and excited states (singlet S0,triplet T1). Horizontal dashed lines represent the molecular vibronicenergies. Here, Δr represents the average of atomic distances within themolecule, and E the energy. The lowest E(Δr) corresponds to theequilibrium state.

If concentrated onto a single molecular bond on the host, the excessenergy can lead to molecular decomposition, or fragmentation, creatingthe non-radiative trap. This is a fundamental energy-driven process. Itis expected the highest energy (blue) excitons lead to the highestenergy polarons with the greatest probability for breaking bonds. Thisis precisely what is observed: blue PHOLEDs typically have orders ofmagnitude shorter lifetimes compared with green and red. This ultimatelydetermines the lifetime of white PHOLEDs that may need to emitapproximately 25% of their light in the blue to generate certain typesof white color.

According to embodiments disclosed herein, the effects of doubly excitedtriplets may be mitigated by “sinking” them with a manager material. Themanager material provides a “solid state sink” energy level, to whichthe doubly excited triplets or polarons may transition instead oftransitioning to the dissociative energy level. As further disclosedherein, this concept may be implemented by doping a blue PHOLEDguest/host system with a second dopant, which removes doubly excitedtriplets and/or polarons from the system by drawing them to groundstate. FIG. 4 shows an example energy level arrangement that may be usedto “sink” the highly excited states before they result dissociativereactions with the host or phosphor dopant, using such a solid statesink energy level. In FIG. 4, the desired outcome for light emission isa transition from the T1 to the S0 state in the blue phosphor. However,this transition competes with triplet-triplet (TTQ) and triplet-polaron(TPQ) annihilation that can promote the exciton to a higher state T2/P2.This higher excitation can result in a destructive chemical reaction(TTQ or TPQ). According to embodiments disclosed herein, when the EML iscodoped with an excited sink dopant, this reaction then competes withthe non-dissociative transition, which returns the molecule from Tx/Pxto T1, or to S0 in the manager molecule.

In such an arrangement, the rate of transition to the manager excitedstate or LUMO Tx/Px should be higher than the rate to the dissociativestate D at which damaging TPQ and TTQ may occur. Thus, to effectivelymanage the multiply excited triplets, both the energetic and spatialproperties of the system should favor the transition from T2/P2 to Tx/Pxover the transition to D. This suggests that the energy of the Tx/Pxstate should be lower than the D state. Alternatively or in addition, itmay suggest that the manager molecules should be positioned relativelyclose to the phosphor through a high doping concentration to allow forrapid energy and/or charge transfer. As previously described, the rateof transition of doubly excited excitons and/or polarons from to thesolid state sink energy level may be higher than the rate of transitionto the dissociative energy level. The relative efficacy of “sinking”need not be perfect, as removing only a fraction of the double excitedstates can have a significant effect on the PHOLED lifetime. Forexample, it has been shown that about only 1in1000 excitations leads tomolecular dissociation that then forms a non-radiative recombinationcenter. Appropriate molecules for the “manager” material include UVabsorbers such as NTCDA 1,4,5,8-Naphthalenetetracarboxylic dianhydride(NTCDA).

More specifically, according to an embodiment disclosed herein, anorganic emissive layer such as a blue emissive layer in a PHOLED orother OLED may include a host material that is doped with at least twodopants. The first dopant, typically the emissive dopant, has adissociative energy level at which doubly excited excitons and/orpolarons undergo a dissociative reaction as previously described, whichmay cause damage to the host molecule. In general, the most damagingreaction is when an exciton on the dopant and a polaron on the hostundergo triplet-polaron quenching, which typically will cause damage tothe host. The first dopant also may have an emissive first tripletenergy level T1 that is lower than the solid state sink energy level ofthe second dopant, as shown in FIG. 4. As described in further detailbelow, the first dopant also may be one that is selected to providethermally assisted delayed fluorescence. More generally, the firstdopant may be a phosphorescent or a fluorescent dopant. The dopant maybe one that emits light of any color, though it is expected that thebenefits described herein will be particularly suited to blue-emittingmaterials.

In some embodiments, the manager material, i.e., a second dopant, mayhave a solid state sink energy level that is between the singly-excitedtriplet or polaron energy level of the emissive material and amultiply-excited triplet or polaron energy level of the emissivematerial, typically a doubly-excited triplet or polaron energy level.For example, referring to FIG. 4, the Tx/Px energy level of the manageris between the T1 and T2/P2 energy levels of the emissive material. Suchan arrangement may provide the benefits disclosed herein even where thesink energy level Tx/Px is not substantially lower than, equal to, orslightly higher than the dissociative energy level D, because competingrates of transition to the two levels may reduce degradation in the hostand/or emissive materials, as disclosed herein. In general, it may bepreferred for the solid state sink energy level to be at least 0.2 eVbelow the multiply-excited triplet or polaron energy level of theemissive material, to prevent transfers back to the multiply-excitedstate. For example, referring to FIG. 4, it may be preferred for thesolid state sink energy level Tx/Px to be at least 0.2 eV lower than thedoubly-excited triplet or polaron energy level T2/P2.

In the annihilation process in FIG. 3, an exciton and polaron combine togenerate “hot” states via:T*+H ⁻ →T ₀ +H ^(−/)*or T*+H ⁻ →T* ^(/−) +H ₀where “*” indicates a multiply excited state, “−” a polaron on thecorresponding molecule, and host (H^(−/)*) or phosphor (T^(−/)*)represent a hot polaron or hot triplet on the host or phosphor,respectively. It may not be possible to quantify this reaction directly,since the amount of degraded material in the thin film typically is toolow to be analytically measured. The hot polaron states may be quantizedsimilarly to the techniques described in I. Ghosh et al., Science 346,725-728 (2014), in which where a reduced organic compound in solutionwas photolyzed to generate the equivalent hot state. This state canreduce chloro-aromatic compounds, ultimately producing halide ions andaryl radicals. A similar approach may be used in which solutions of T⁻and H⁻ ions are irradiated while monitoring their photostability todetermine whether a hot polaron on the phosphor or host is the “weaklink” in a given phosphor-host pair. The excited state lifetimes ofH^(−/)* and T*^(/−) may be characterized using transient absorption (TA)spectroscopy, and the hot polaron lifetime correlated with itsphotostability. In addition, high performance liquid chromatography,mass spectroscopy and Fourier transform infrared spectroscopy may beused to identify the decomposition products. With this information,“hardened” materials may be designed that are less susceptible tofragmentation in the H^(−/)* or T*^(/−) state, further decreasing thelikelihood of destructive decay from the hot polaron.

A similar approach may be used to estimate the energy of H^(−/)* orT^(−/)* by examining the photostability of H⁻ and T⁻ in the presence ofa range of trapping agents. By using trapping agents with gradedreduction potentials the reducing power of the hot polaron may beestimated. This estimate then allows for a selection of appropriatecandidates from libraries of H3 and H2P materials, for example as shownin FIG. 5, to quench the hot polaron, while not trapping charge ortriplets. FIG. 5A shows a structure suitable to measure aza-substitutionin an H3P material; the variation provides 37 different compounds, asshown in FIG. 5B. Other materials also may be used as triplet-tripletand triplet-polaron quenchers, such as pyridyl, pyrazine and triazinebased materials which are reducible, stable, and have high tripletenergies.

The described approach is focused on the anionic host and dopant. Thisis because in most cases the phosphor dopant traps and transports holes,while electrons are transported by the host matrix. Hence, the electrondensity in the EML tends to significantly exceed that of higher mobilityholes. Nevertheless, the methodology can be used equally well to studythe stability of cathodic H^(+/)* and T^(+/)* states if the primarycarrier in the host is the hole.

As previously described, the model illustrated in FIG. 3 suggests thatPHOLED operational lifetimes can be greatly extended by reducingtriplet-polaron quenching (TPQ) and triplet-triplet quenching (TTQ)annihilation events that lead to molecular destruction. Even removingonly a small fraction of the highly excited states may have a remarkableeffect on PHOLED lifetime since there is a probably of only 1 in 2×10⁻⁹that an excitation will lead to molecular dissociation. Thus,alternatively or in addition to using a manager material having a solidstate sink energy level as previously described, in embodimentsdisclosed herein, the rate of TPQ and TTQ may be reduced throughextending the device emission zone via phosphor dopant grading acrossthe PHOLED emissive layer. That is, the concentration of a dopant withina host-dopant system as previously described with respect to FIG. 4 maybe graded within the host material. Such a configuration may provide upto a tenfold improvement in blue PHOLED operational lifetime relative toconventional designs. Strategic grading reduced the exciton and polarondensities (and hence their tendency to create traps through energeticencounters) while also increasing efficiency at high brightness byreducing triplet annihlation. This approach has been found to triple thelifetime of a relatively stable blue emitting phosphor from LT₈₀=50 hrsat an initial luminance of L₀=1000 cd/m². When used in a stackedstructure, a lifetime of LT₈₀>600 hr may be achieved. In this example,the phosphor Ir(dmp)₃, a hole conductor, is paired with a stable,electron conducting host, such as mCBP. Ordinarily, the doping of thePHOLED EML is uniform, in which case almost all excitons form at theEML/electron transport layer (ETL) interface, resulting in the localpile-up of a high density of electrons and excitons where TPQ and TTQ isfrequent. To lower local exciton and polaron densities, the phosphorconcentration may be graded from, for example, about 20% at the holetransport layer (HTL)/EML interface, to about 8% at the opposite EML/ETLinterface. For example, an emissive layer as described with respect toFIGS. 1-2 may be graded between two adjacent layers, such that theconcentration of a phosphorescent or other emissive dopant in theemissive layer is graded vertically across the emissive layer. Such astructure has been found to increase the lifetime of a blue PHOLED by afactor of up to 10 when used in a stacked blue PHOLED.

Different grading profiles from those used to optimize simple guest-hostdoped EMLs may be used, since the presence of the sink molecule maychange the exciton and charge distributions. It is anticipated that thecombination of graded doping and the use of excited state sinks canincrease the lifetime of blue PHOLEDs by a factor of 1000 or more,compared to current conventional devices. This approach is general, andhence can also be applied to red and green phosphorescent devices aswell as blue devices as disclosed herein. Hence, excited state sinkingmay result in phosphor lifetimes exceeding 1 million hours in somecases.

Excited state management as disclosed herein also may be used in devicesand arrangements that make use of phosphor sensitized fluorescenceand/or thermally activated delayed fluorescence (TADF). Both techniquestypically require sensitizers having energies higher than the emissiveenergy, especially “hyperfluorescence” in which a TADF sensitizertransfers energy to a blue fluorophore. In both cases, EML gradingand/or excited state management as disclosed herein may provide improvedlifetimes of the devices. For example, the first or primary dopant in aguest/host emissive layer as disclosed herein may be a dopant suitablefor use with TADF, in which excitons are maintained in the tripletmanifold of the dopant. A UV sensitizer is then used to access thefluorescence dopant energies. However, the longer triplets aremaintained at a relatively high energy, the higher the chance that theycollide with a triplet or polaron from another molecule and undergo adestructive transition as previously described. The excited statemanagement techniques disclosed herein may prevent such destructivetransitions, thereby improving the lifetime of the TADF or similarsystem.

Techniques disclosed herein may be suitable for use in any arrangementor device that includes organic emissive layers, such as OLEDs,white-emitting OLEDs (WOLEDs), and the like. Similarly, any device thatcan incorporate an OLED as disclosed herein, also may incorporate OLEDshaving emissive layers as disclosed herein.

Layers, arrangements, and techniques as disclosed herein may be usedwith any device that uses organic emissive layers, such as the OLEDsdescribed with respect to FIGS. 1-2. For example, one or more emissivelayers in an OLED, such as emissive layers 135, 220, may be doped withfirst and second dopants as disclosed herein to provide a solid statesink energy level within the emissive layer as previously described.Such a device may be a pixel or sub-pixel in a device such as afull-color display, or any other device that incorporates structuressuch as shown in FIGS. 1-2 and disclosed herein. It is understood thatthe various embodiments described herein are by way of example only, andare not intended to limit the scope of the invention. For example, manyof the materials and structures described herein may be substituted withother materials and structures without deviating from the spirit of theinvention. The present invention as claimed may therefore includevariations from the particular examples and preferred embodimentsdescribed herein, as will be apparent to one of skill in the art. It isunderstood that various theories as to why the invention works are notintended to be limiting.

We claim:
 1. A device comprising: a first electrode; a second electrode; an organic emissive stack disposed between the first electrode and the second electrode, the organic emissive stack comprising a first emissive layer, the first emissive layer comprising: a host material; a first dopant that provides thermally assisted delayed fluorescence (TADF) in the host material and has a dissociative first energy level at which doubly-excited excitons and/or polarons undergo a dissociative reaction, and a second dopant having a solid state sink energy level that is between a singly-excited energy level of the first dopant and a multiply-excited energy level of the first dopant, and lower than the dissociative first energy level of the first dopant.
 2. The device of claim 1, wherein the concentration of the first dopant and/or the second dopant is graded within the host material.
 3. The emissive layer of claim 1, wherein the multiply-excited energy level of the first dopant is a doubly-excited triplet energy level.
 4. The emissive layer of claim 1, wherein the multiply-excited energy level of the first dopant is a doubly-excited polaron energy level.
 5. The emissive layer of claim 1, wherein the first dopant has an emissive first triplet energy level that is lower than the solid state sink energy level of the second dopant.
 6. The emissive layer of claim 1, wherein the solid state sink energy level is at least 0.2 eV below the multiply excited energy level.
 7. The emissive layer of claim 1, wherein the concentration of the first dopant is graded within the host material.
 8. The emissive layer of claim 1, wherein the rate of transition of the excitons and/or polarons from a doubly-excited triplet energy level of the first dopant to the solid state sink energy level of the second dopant is higher than the rate of transition from the double-excited triplet energy level of the first dopant to the dissociative energy level.
 9. The emissive layer of claim 1, wherein the second dopant absorbs light in the ultraviolet region of the electromagnetic spectrum.
 10. The emissive layer of claim 9, wherein the second dopant comprises 1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTCDA).
 11. The device of claim 1, wherein the device comprises a white-emitting OLED (WOLED).
 12. The device of claim 11, wherein the first emissive layer emits light in the blue region of the visible spectrum.
 13. The device of claim 12, wherein the organic emissive stack further comprises a second emissive layer that emits light in the green or red region of the visible spectrum.
 14. A consumer electronic product comprising: a display comprising: a first electrode; a second electrode; an organic emissive stack disposed between the first electrode and the second electrode, the organic emissive stack comprising a first emissive layer, the first emissive layer comprising: a host material; a first dopant that provides thermally assisted delayed fluorescence (TADF) in the host material and has a dissociative first energy level at which doubly-excited excitons and/or polarons undergo a dissociative reaction, and a second dopant having a solid state sink energy level that is between a singly-excited energy level of the first dopant and a multiply-excited energy level of the first dopant, and lower than the dissociative first energy level of the first dopant.
 15. The consumer electronic product of claim 14, wherein the consumer electronic product is selected from the group consisting of: a flat panel display, a computer monitor, a medical monitor, a television, a billboard, a light for interior or exterior illumination and/or signaling, a heads-up display, a fully or partially transparent display, a flexible display, a laser printer, a telephone, a cell phone, a tablet, a phablet, a personal digital assistant (PDA), a laptop computer, a digital camera, a camcorder, a viewfinder, a micro-display, a 3-D display, a smart watch, a vehicle, a large area wall, theater or stadium screen, and a sign. 